This invention relates to apparatus for evaluation of the current versus voltage (I-V) parameter of solar cells, and more particularly to a circuit for varying the load aplied to a solar cell between short circuit and open circuit while simultaneously driving an X-Y recorder for display of the I-V curve of solar cells.
An energy shortage has stimulated a great deal of effort to develop more efficient solar photovoltaic cells. One of the important evaluation parameters of such cells is the current versus voltage (I-V) parameter displayed by sweeping the cell from open circuit to short circuit under standardized illumination, and displaying the current output of the cell as a function of load voltage.
The present method of generating such a parameter display is to connect the solar cell in series with a resistor R to a variable voltage source. The voltage developed across the resistor is a function of current through the solar cell for a given voltage from the source. The voltage across the resistor is therefore used as the Y (current) input to an X-Y plotter while the voltage across the solar cell is used as the X input to the plotter. A potentiometer is employed to vary the applied voltage for a given light level on the solar cell. Usually the voltage source consists of batteries capable of providing the anticipated test current at the desired test voltage, with the potentiometer placed across the center-tapped batteries so that the potentiometer arm voltage may be swept from negative (cell voltage zero) to positive (cell current zero), and vice versa.
This present method is not entirely satisfactory because to a large degree testing depends upon the skill of the operator and the care exercised. Testing is therefore somewhat of an art. Consequently curves for the same or similar cells obtained by various operators, differ. In part this is due to the conflicting requirements that potentiometer movement be rapid enough to prevent build-up of heat in the aluminated cell (which affects cell output and hence the test data), and yet slow enough to allow the recorder to follow the changing cell output faithfully, particularly around and past the knee of the curve.
Another problem arises from the difficulty of determining the location of end points V.sub.oc and I.sub.sc on the curve rapidly, as soon as the cell is illuminated and before heat build-up can occur. Since the I-V parameter is different when the cell is dark, the operator must estimate a suitable potentiometer position before the beginning of the test. It is desirable that the potentiometer be set initially so that the cell voltage is equal to, or slightly less than zero, but this cannot be determined until the cell is actually illuminated. When a setting is made initially, and the cell is then illuminated, it may be found that the resulting cell voltage is positive, in which case it is necessary to decrease the potentiometer setting until cell voltage is zero before sweeping from zero to V.sub.oc in order to obtain the I-V curve. On the other hand, it may be found that the cell voltage is so far negative that the recorder is overrunning past the Y axis. When the potentiometer setting is increased for the test it is difficult to estimate just how fast to move the potentiometer. The beginning of the curve may then include an initial jerky portion. These and like situations may make it necessary to rerun the test, and it thus may require a number of runs to obtain a suitable test curve result. The test can be made in the reverse direction instead, (i.e. with an initial potentiometer setting which will yield a cell current equal to, or slightly less than, zero) but exactly analogous problems arise in estimating the potentiometer setting. An automated test system which determines the end points quickly at the start of the test, and which provides a test voltage which varies at a suitable rate so that the recorder follows the changing output data faithfully, would produce more uniform test results regardless of operator skill, and in less test time to prevent build-up heat from affecting the test.